(1) Field of the Invention
This invention relates to the recovery of solvents which are absorbed on an adsorbent from exhaust air containing solvent vapors.
(2) Description of the Prior Art
Process and apparatus for recovering solvents are known (see, e.g., Ullmans Encyklopadie der technischen Chemie 4th Edition, 1972, Vol. 2, page 613).
The best known process for solvent recovery operates with granular active carbon in a fixed bed in vertical or horizontal adsorbers. The carbon is regenerated with steam as the desorption medium, which flows through the adsorber counter to the loading direction. After the treatment with hot steam, the adsorber is dried with air and cooled. For cooling, the exhaust air laden with solvent vapors can be used, i.e., the process of drying and cooling can be coupled with loading.
Desorption of the adsorbent with steam has certain disadvantages, however.
In the first place, the energy required to generate the steam cannot be recovered. Further energy, in the form of hot air, is required for drying the adsorbent. Certain adsorbents, such as silica gel or molecular sieves, are very hard to dry at all. When active carbon is used as adsorbent, the drying air must not be too hot in order to prevent self-ignition of the active carbon or of the solvent residues still adsorbed on it. On the other hand, if considerably cooler air is used the required drying time is prolonged.
A further disadvantage is that the recovered solvents have to be separated from the steam, which is of considerable difficulty when the solvents are water soluble and their boiling points lie near that of water. In this case, an expensive rectification plant is necessary for the separation.
Furthermore, certain solvents, in some circumstances because of the catalytic action of the adsorbent, react with steam or with the hot drying air; e.g., in the case of chlorinated hydrocarbons, hydrogen chloride is liberated and causes corrosion in the plant.
The object of the present invention is to provide a process and an apparatus of the type defined above, by means of which the solvents can be desorbed and separated from the desorbent in a simple and easily controllable manner, with a considerably lower expenditure of energy, and without decomposition phenomena.